Retaining rings are commonly fixed to end parts of the coils or windings of rotors of high power electric machines. The retaining rings mainly serve to mechanically stabilize the end parts, also referred to as end windings. The main parts of the windings are housed in the rotor slots, the end windings of the rotor however protrude outside of the slots and are in the case of high power generators bent in relation to the main winding part. To this end the retaining ring is fabricated as massive cylinder which encloses the end windings. Some retaining rings are internally insulated. The insulation is an electric insulation and consists of several layers of fibreglass with a layer thickness in the range of millimeters. The insulation layers can further be teflonized, i.e. a layer of polytetrafluoroethylene can be applied. The insulation layers are attached at the inside of the retaining ring by gluing with hot curing resin. Presently, with the need to replace the insulation of the retaining ring expensive tools are used. These tools are expansible steel cones or cylinders depending on the geometry of the retaining ring. The expansible steel cones or cylinders exert a pressure to the inside of the retaining ring with applied insulation. This high pressure is necessary to obtain a sufficient bond between the two different materials of the retaining ring and the insulation.